Hydrogel with selective absorption for separation of liquid mixtures

ABSTRACT

Methods for synthesizing a hydrogel are disclosed. The method includes the steps of: (a) dissolving effective amounts of a monomer, a crosslinker, and a photoinitiator in deionized water, wherein the overall concentration is about 200 mg/ml; (b) preparing a solution of a fluorinated acrylate or diacrylate in ethanol wherein the overall concentration is about 200 mg/ml; (c) separately stirring the solutions prepared in steps (a) and (b) for approximately three hours; (c) gradually introducing the solution from step (b) into the solution from step (a); (d) stirring the resulting solution from step (c) for about two hours; and (e) exposing the solution from step (d) to UV-A irradiation for about 15 minutes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/629,356 filed Feb. 12, 2018, the entirety of which isincorporated herein by reference.

BACKGROUND

Separation operations of liquid mixtures (immiscible or miscible) spanacross numerous manufacturing industries including petrochemicals,textiles, leather, wastewater treatment and biofuel production.According to a recent report, separation operations account for aquarter of all in-plant energy consumption in the United States.

One of the most ubiquitous immiscible liquid mixtures is oil and water.A large amount of oily wastewater is produced every day duringindustrial processing. The effect of this wastewater on the environmentcan be severe, unless it is adequately treated before discharge, as canbe observed from the many recent oil-spill disasters. In addition,shortage of freshwater has become a severe problem in the world,especially in certain underdeveloped regions. Purification of oilywastewater can enhance the amount of water available for use.Furthermore, expulsion of water from fuel oil is of great concern inpetroleum and automobile industries because even a small amount of waterin the fuel oil may damage engines, threatening the safety of theautomobile.

The separation of miscible liquid mixtures is also important in manyindustries. For example, in the petroleum refining process, smallamounts of miscible impurities including sulfur, nitrogen and metalcompounds are separated from crude oil to produce fuel oil. Similarly,the high quality of biofuels such as bioethanol or biodiesel can only beproduced by removing dissolved byproducts generated during theseparation process. In addition, recovery of organic acids fromagroindustrial wastewater is essential not only for environmentalrequirements, but also for economic benefits.

A large number of methodologies including distillation, liquid-liquidextraction and membranes have been used to separate miscible orimmiscible liquid mixtures. Distillation separates components from amixture based on differences in their boiling points. Since distillationis a simple and well-established technology, it is by far the mostwidely used separation process. However, distillation has low energyefficiency and it requires thermal stability of compounds at theirboiling points. In addition, it is not suitable for the separation ofcomponents with similar boiling points such as azeotropes.

Liquid-liquid extraction is typically used to separate azeotropes andcomponents with overlapping boiling points where simple distillationcannot be used. Liquid-liquid extraction is a separation technique thatseparates components of a liquid mixture by contact with anotherinsoluble liquid. Components in a liquid mixture are separated based ontheir difference in solubility with the insoluble liquid. One primarychallenge in liquid-liquid extraction is to increase contact between thetwo liquid phases for efficient mass transfer. This is typicallyachieved by employing energy-intensive ultrasonication or pumping thetwo liquids through packed columns with high tortuosity.

Membrane-based technologies physically separate a liquid mixture intoits components by allowing one phase to permeate through the membranewhile retaining the other component. Since the separation is performedat ambient temperature without chemically altering the components,membrane-based separation operations consume less energy than otherseparation methods. However, membranes can be fouled by particulates ororganic matters during the separation operation, which results in adecline of the permeability.

Absorption can be a useful alternative for the separation of eithermiscible or immiscible liquid mixtures. Embodiments of absorptiontechniques are described herein for the purpose of separating miscibleand/or immiscible liquid mixtures.

SUMMARY

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is notintended to identify the critical elements of the invention or todelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented herein.

In one embodiment, a hydrogel has effective amounts of:N-ispropylacrylamide (NIPAM); N,N′ Methylenebisacrylamide (MBAA);2-hydroxy-2-methylpropiophenone; and 1H,1H,2H,2H-perfluorodecylacrylate. The hydrogel has the following chemical structure:

In another embodiment, a method for synthesizing a hydrogel includes thesteps of: (a) dissolving N-isopropylacrylamide, N,N′Methylenebisacrylamide (MBAA) and 2-hydroxy-2-methylpropiophenone indeionized water to form a NIPAM solution; (b) preparing a solution of1H,1H,2H,2H-Perfluorodecyl acrylate in ethanol; (c) separately stirringthe solutions prepared in steps (a) and (b) for approximately threehours; (c) gradually introducing the solution from step (b) into thesolution from step (a); (d) stirring the resulting solution from step(c) for about two hours; and (e) pouring the resulting solution fromstep (d) into a mold and irradiating with UV-A (λ=365 nm) for about 15minutes.

In still another embodiment, methods for synthesizing a hydrogel aredisclosed. The method includes the steps of: (a) dissolving effectiveamounts of a monomer, a crosslinker, and a photoinitiator in deionizedwater, wherein the overall concentration is about 200 mg/ml; (b)preparing a solution of a fluorinated acrylate or diacrylate in ethanolwherein the overall concentration is about 200 mg/ml; (c) separatelystirring the solutions prepared in steps (a) and (b) for approximatelythree hours; (c) gradually introducing the solution from step (b) intothe solution from step (a); (d) stirring the resulting solution fromstep (c) for about two hours; and (e) exposing the solution from step(d) to UV-A irradiation for about 15 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration the synthesis of F-PNIPAM.

FIG. 2 is a plot showing Fourier-transform infrared spectroscopyabsorption spectrum for NIPAM and F-PNIPAM.

FIG. 3A is an EDS elemental spectrum on F-PNIPAM.

FIG. 3B is an EDS elemental mapping of fluorine, wherein the scale barrepresents 100 micrometers.

FIG. 4 is a photograph showing the contact angles for ethanol, water,hexadecane, and heptane droplets on a F-PNIPAM coated surface at roomtemperature (T=21° C.).

FIG. 5 is a photograph showing the contact angles for ethanol, water,hexadecane, and heptane droplets on a F-PNIPAM coated surface atelevated temperature (T=40° C.).

FIG. 6 is a plot showing contact angles of water and various oils on thesurface of F-PNIPAM as a function of wt. % of F-acrylate below LCST(T=21° C.).

FIG. 7 is a plot showing contact angles of water and various oils on thesurface of F-PNIPAM as a function of wt. % of F-acrylate above LCST(T=40° C.).

FIG. 8 is a graph showing wettability for water and hexadecane contactangles through repeated cycles of heating and cooling.

FIG. 9 is a plot showing DSC data for various concentrations ofF-acrylate.

FIG. 10 is a plot showing the surface energy of F-PNIPAM as a functionof wt. % F-acrylate, wherein γ_(SV) ^(d) is the dispersive component,γ_(SV) ^(p) is the polar component, and γ_(SV) is the surface energy.

FIG. 11 is a graph showing normalized uptake of liquids as a function ofF-PNIPAM weight.

FIG. 12 is a graph showing normalized uptake of liquids as a function ofcrosslinker concentration in F-PNIPAM and PNIPAM.

FIG. 13 is a graph showing normalized uptake of various liquids as afunction of submerged time.

FIGS. 14A and 14B are photographs of separation of free hexadecane-watermixture using F-PNIPAM.

FIG. 14C is a photograph where almost all water is absorbed from themixture shown in FIGS. 14A and 14B.

FIG. 15A is a photograph of F-PNIPAM submerged in a miscibleethanol-heptane mixture.

FIG. 15B is a photograph of the F-PNIPAM after 40 minutes in solution,swollen due to selective absorption of ethanol.

FIG. 16 is a graph showing refractive indices as a function of heptanevol % in ethanol-heptane 4 mixtures.

FIG. 17 is a plot showing equilibrium swelling ratio of F-PNIPAM inpolar and non-polar solvents, wherein MeOH is methanol, EtOH is ethanol,HpOH is heptanol, HT is heptane, and HD is hexadecane.

FIG. 18 is a plot showing equilibrium swelling ratio of F-PNIPAM as afunction of MBAA composition.

FIG. 19 is a plot showing equilibrium swelling ratio of F-PNIPAM as afunction of polymer mass. FIG. 19 shows that equilibrium swelling ratiofor a given liquid is not significantly altered by polymer mass.

FIG. 20 is a plot showing equilibrium swelling ratio of F-PNIPAM foralcohols with various number of hydrocarbons, wherein MeOH is methanol,EtOH is ethanol, PpOH is propanol, BuOH is butanol, PtOH is pentanol,HxOH is hexanol, and HpOH is heptanol.

FIG. 21 is a plot showing x values of F-PNIPAM for alcohols with variousnumbers of hydrocarbons, wherein MeOH is methanol, EtOH is ethanol, PpOHis propanol, BuOH is butanol, PtOH is pentanol, HxOH is hexanol, andHpOH is heptanol.

FIG. 22 is a plot showing the swelling ratio of F-NIPAM in water as afunction of time.

FIG. 23 is a plot showing swelling ratio (S.R.) of F-PNIPAM in alcoholswith different number of hydrocarbons as a function of time.

FIGS. 24A-D are a series of photographs showing the separation of a freehexadecane-water solution.

FIG. 25 is a plot showing the separation efficiency for free hexadecaneand water as a function of time using F-PNIPAM.

FIG. 26 is a thermogravimetric analysis (TGA) plot for the remnant afterseparation of free hexadecane and water. The plots for hexadecane andwater are also included.

FIG. 27 is a plot showing swelling percentage as a function of submergedtime when the submerged area of F-PNIPAM in water is varied.

FIG. 28 is a plot showing size distribution of hexadecane in sodiumdodecylsulfate (SDS) stabilized hexadecane-in-water (30:70 vol:vol)emulsion. The inset shows an optical microscope image of the emulsion;the bar represents 100 micrometers.

FIG. 29 are photographs showing separation of hexadecane-in-water (30:70vol:vol) emulsion. In (a) F-PNIPAM is submerged in the emulsion. (b)represents a time lapse of 15 minutes, showing the F-PNIPAM swollen withwater from the emulsion. In (c), the F-PNIPAM is removed, and theremnant is almost pure hexadecane.

FIG. 30 is a TGA plot for the remnant after separation ofhexadecane-in-water (30:70 vol:vol) emulsion.

FIG. 31 is a plot showing separation efficiency of F-PNIPAM forhexadecane-in-water (30:70 vol:vol) emulsion as a function of time.Separation of efficiency for hexadecane of 50:50 vol:vol emulsion isalso shown.

FIG. 32 is a plot showing size distribution of water in span80stabilized water-in-hexadecane (50:50 vol:vol) emulsion. The insert isan optical microscope image of the emulsion, with the scale barrepresenting 100 micrometers.

FIG. 33 are photographs showing separation of water-in-hexadecane (50:50vol:vol) emulsion. In (a) F-PNIPAM is submerged in the emulsion. (b)represents a time lapse of 30 minutes, showing the F-PNIPAM swollen withwater from the emulsion. In (c), the F-PNIPAM is removed, and theremnant is almost pure hexadecane.

FIG. 34 is a TGA plot for the remnant after separation ofwater-in-hexadecane (50:50 vol:vol) emulsion.

FIG. 35 is a plot showing separation efficiency of F-PNIPAM forwater-in-hexadecane (50:50 vol:vol) emulsion as a function of time.

FIG. 36 shows a series of photographs showing separation ofheptane-ethanol azeotrope using the F-PNIPAM. In photograph (a),F-PNIPAM (˜3 cm³) is submerged into the azeotrope. In (b), after 60minutes, the F-PNIPAM has absorbed the ethanol. In (c), the F-PNIPAM isremoved, leaving only the heptane.

FIG. 37 is a plot showing a refractive index of the heptane-ethanolmixture as a function of volume percentage of heptane. The refractiveindex of heptane-ethanol azeotrope is shown both before and afterseparation.

FIG. 38 is a plot showing separation efficiency for heptane-ethanolazeotrope as a function of submerged time.

FIG. 39 shows a series of images showing selective absorption of ethanolwhile heptane-ethanol azeotrope slides along the surface of F-PNIPAM. In(a), the drop begins to slide along the F-PNIPAM surface. In (b), theethanol is absorbed, while the azeotrope slides, as indicated by thechange in the color of the F-PNIPAM surface. In (c), the sliding iscompleted, and the heptane is collected.

FIG. 40 is a series of photographs showing the separation of methanoland methyl oleate (30:70 vol:vol). In photograph (a), F-PNIPAM (˜3 cm³)is submerged into the mixture. In (b), after 60 minutes, the F-PNIPAMhas absorbed the methanol. In (c), the F-PNIPAM is removed, leaving onlythe methyl oleate.

FIG. 41 is a plot showing separation efficiency for methanol-methyloleate mixture as a function of time.

FIG. 42A is a bar graph showing the volume ratio of liquid absorbed byF-PNIPAM as a function of the ethanol:water (vol:vol) ratio below LCST(T=21° C.). 96.5:3.5 corresponds to the composition of ethanol-waterazeotrope. For each concentration, the bar on the left representsethanol, while the bar on the right represents water.

FIG. 42B is a bar graph showing the volume ratio of liquid absorbed byF-PNIPAM as a function of the ethanol:water (vol:vol) ratio above LCST(T=40° C.). 96.5:3.5 corresponds to the composition of ethanol-waterazeotrope. For each concentration, the bar on the left representsethanol, while the bar on the right represents water.

FIG. 43A is a bar graph showing the volume ratio of liquid absorbed byF-PNIPAM as a function of the DMF:water (vol:vol) ratio below LCST(T=21° C.). For each concentration, the bar on the left representsdimethylformamide (DMF), while the bar on the right represents water.

FIG. 43B is a bar graph showing the volume ratio of liquid absorbed byF-PNIPAM as a function of the DMF:water (vol:vol) ratio above LCST(T=40° C.). For each concentration, the bar on the left represents DMF,while the bar on the right represents water.

FIG. 44 is a photograph of F-PNIPAM after absorbing water, and afterreleasing water.

FIG. 45 is a graph showing normalized water uptake of F-PNIPAM afterrecovery processes.

FIG. 46 is a graph showing normalized ethanol uptake of F-PNIPAM afterrecovery processes.

FIG. 47 is a series of photographs showing the release of water fromF-PNIPAM with mild heat applications (T=33° C.). In (a), the F-PNIPAM isswollen to equilibrium by absorbing water at T=21° C. In (b), theF-PNIPAM released 82% of absorbed water at the heat applicationtemperature. Finally, in (c), the F-PNIPAM is shrunken to about itsas-prepared state.

FIG. 48 is a plot showing water recover from F-PNIPAM as a function oftime in various concentration of aqueous NaCl solution.

FIG. 49 is a plot showing ethanol recovery from F-PNIPAM as a functionof time in various concentrations of aqueous NaCl solution.

FIG. 50 shows a series of photographs showing oil fouling on the surfaceof hydrophilic/oleophilic neat NIPAM. In (a), the NIPAM is fouled byhexadecane. In (b), the oil fouling hinders the ability of water (thedarker colored spot) to wet the surface.

FIG. 51A is a schematic illustrating an apparatus for separating anoil-water mixture according to an embodiment of the invention.

FIGS. 51B and C are photographs showing the continuous separation of anoil-water mixture utilizing the apparatus of FIG. 51A.

FIG. 52A is a schematic illustrating a separation apparatus forseparating an oil-water mixture according to another embodiment of theinvention.

FIGS. 52B and C are photographs showing the continuous separation of anoil-water mixture utilizing the apparatus of FIG. 52A.

DETAILED DESCRIPTION

Described herein are hydrogels with selective wettability of water(polar liquid) over oil (non-polar liquid) and methods for synthesizingsame. As is described in greater detail below, the hydrogels set forthherein may selectively absorb polar liquid while repel non-polar liquid.Utilizing such a highly selective absorption behavior, it may bepossible to almost completely separate both immiscible oil-watermixtures and miscible polar-non-polar liquid mixtures.

A hydrogel is a three-dimensional polymer that can hold water in itsnetwork when submerged in water. Due to such a unique ability to absorbwater, hydrogels have been widely studied and applied in a range ofapplications. Common examples of hydrogels are polyethylene glycol(PEG), polyvinyl alcohol (PVA), hydroxypropyl cellulose (HPC),polyacrylamide (PAM), etc.

Poly N-isopropylacrylamide (PNIPAM) is a thermo-responsive hydrogel thatcan show different water absorption behavior as a function oftemperature. For example, PNIPAM readily gets wet by water (that is,hydrophilic) and absorbs it at a relatively lower temperature (T<32°C.). At an elevated temperature (T>32° C.), PNIPAM becomes hydrophobic(that is, repellent to water) and releases water retained within itsnetwork. The temperature (T=32° C.) at which PNIPAM undergoes thetransition between absorbing and releasing water is known as lowercritical transition temperature (LCST). Such a thermo-responsiveswitching of water-wettability of PNIPAM has led to several reports ofseparating oil-water mixtures upon heating and cooling. However, thesereports often failed to characterize the wettability (or absorption) ofoils by the PNIPAM. Almost all hydrogel is hydrophilic and oleophilic(wet by oil) unless it is carefully designed and synthesized to possessselective wettability. The hydrogels described herein uniquely exhibitin-air hyrophilicity and oleophobicity.

Liquids can be considered either polar or non-polar. Polar liquids (suchas water, alcohols and acetone) possess a dipole in their molecules,while non-polar liquids (such as alkanes, benzene, toluene, etc.) donot. Typically, polar (e.g., water) and non-polar (e.g., oil) liquids donot mix together. They form two separate phases when they contact eachother. However, there are a few pairs of polar and non-polar liquidsthat are completely miscible.

One of the well-known examples of an immiscible liquid mixture is oiland water. Oil-water mixtures are classified, in terms of the diameter(d) of the dispersed phase, as free oil and water if d>150 μm, adispersion if 20 μm<d<150 μm, or an emulsion if d<20 μm. When asurfactant is introduced, the stability of an oil-water mixture issignificantly enhanced. This is because surfactant can effectively lowerthe interfacial tension (γ_(ow)) of oil and water. Examples ofsurfactants that are typically used to stabilize oil-water emulsions (ordispersions) include but are not limited to sodium dodecylsulfate (SDS),Sorbitan monooleate (e.g., Tween80, Span80, Span120, etc.), and Sorbitanmonolaurate (e.g., Tween20, Span20, Tween21, etc.).

In some cases, polar and non-polar liquids can completely mix togetherto form a homogeneous single phase. Examples of such miscible mixturesare ethanol-heptane, butanol-hexane and methanol-diesel fuel. Thesemiscible liquids can be found in various industrial process includingbiodiesel production, jet fuel purification and pharmaceutical process.

Distillation is widely used to separate the miscible liquid mixtures.However, the disadvantages of distillation include its low energyefficiency and that it requires thermal stability of compounds at theirboiling points. Large energy savings could be obtained by replacingdistillation with low-energy intensity operations, such as absorption.

Synthesis of F-PNIPAM Hydrogel

In one example of the invention, a simple free radical polymerizationmethod initiated with ultraviolet (UV) light to synthesize fluorinatedPNIPAM (F-PNIPAM) was employed. First, N-isopropylacrylamide (NIPAM), N,N′ Methylenebisacrylamide (MBAA), and Darocur1173(2-hydroxy-2-methylpropiophenone) were dissolved in deionized (DI) waterin 97:1:2 weight ratio (NIPAM solution). NIPAM, MBAA and Darocur1173 aremonomer, crosslinker and photoinitiator, respectively. The overallconcentration of the solution is about 200 mg/ml.

A solution of 1H,1H,2H,2H-Perfluorodecyl acrylate (perfluoro acrylatesolution, or F-acrylate) was separately prepared in ethanol withsubstantially the same concentration (e.g., about 200 mg/ml). Theprepared solutions are then separately stirred for about 3 hours using amechanical stirrer in dark conditions to prevent light exposure andunexpected cross-linking. Subsequently, perfluoro acrylate solution isgradually introduced to the NIPAM solution, followed by a vigorousstirring for about 2 hours. The ratio of 1H,1H,2H,2H-Perfluorodecylacrylate and NIPAM is maintained at 9:1. The solution was then pouredinto a cubical mold (1.2 cm×1.2 cm) followed by UV-A (λ=365 nm)irradiation for 15 mins. This leads to photoinitiation of free radicalpolymerization and crosslinking.

It shall be noted that 1H,1H,2H,2H-Perfluorodecyl acrylate (F-acrylate)can be replaced by other fluorinated acrylates (or diacrylates)including but not limited to 1H,1H,6H,6H-Perfluoro-1,6-hexandioldiacrylate, 1H,1H-perfluoro-n-octyl methacrylate, and1H,1H-perfluoro-n-octyl acrylate, all of which are contemplated withinthe scope of the invention.

FIG. 1 shows a schematic illustration of synthesizing F-PNIPAM. Upon UVirradiation, Darocur1173 forms radicals that can initiate the bondcleavage of MBAA, NIPAM and perfluoro acrylate. Here it is believed thatNIPAM and perfluoro acrylate are copolymerized whereas MBAA crosslinksthe polymers. After 15 mins of UV irradiation, a three-dimensionalpolymer network of F-PNIPAM is obtained.

Fourier-transform infrared spectroscopy (FTIR) was used to identify thechemical structure of F-PNIPAM and to ensure the copolymerization ofPNIPAM with F-acrylate. Before conducting the FTIR analysis, a F-PNIPAMfilm, prepared by drop casting on a glass slide, was dried to remove thewater vapor present in the film. The sample was then scanned at the rateof 5 cm⁻¹ resolution and the absorption peaks were monitored andcompared with the absorption peaks of NIPAM and F-acrylate. FIG. 2 is aplot showing FTIR absorption spectrum for NIPAM (top line) and F-PNIPAM(bottom line).

The peaks between 1,200 cm⁻¹ and 1,250 cm⁻¹ in F-PNIPAM spectrum in FIG.2 are due to the CF₂ group from F-acrylate. Similarly, due to thepresence of CF₂—CF₃ end group from F-acrylate, an absorption peak isobserved at around 1,153 cm⁻¹. The absorption peak at around 1,741 cm⁻¹is attributed to the presence of C═O stretching from F-acrylate. Thisconfirms that NIPAM and F-acrylate are copolymerized resulting in thechemical structure shown in FIG. 1. The characteristic peak at 3,400cm⁻¹ due to the intense NH stretching is lowered in the F-PNIPAM,indicating that NIPAM and F-acrylate are copolymerized.

Energy Dispersive X-ray Spectroscopy (EDS) was performed to determinethe surface chemistry of F-PNIPAM. EDS was performed in conjunction withScanning Electron Microscope (SEM). FIGS. 3A and 3B show the EDSelemental analysis and mapping, respectively, on the F-PNIPAMcopolymerized with 10 wt. % of F-acrylate. In FIG. 3A, the insert showsthe elemental spectrum zoomed in to highlight fluorine. In FIG. 3B, thespots indicate the fluorine elements. The F-PNIPAM samples weresputter-coated with 20 nm of gold to prevent charging. Here, the entiresurface of the F-PNIPAM is covered by fluorine (F). Fluorine originatesfrom CF₂ and CF₃ groups of F-acrylate.

Wettability

The wettability switch of the F-PNIPAM with a change in ambienttemperature was studied. First, a thin film of 10 wt. % F-PNIPAM wasfabricated on a small piece of glass. Contact angles (θ) for water,ethanol, hexadecane, and heptane were determined at room temperature(T=21° C.) and at elevated temperature (T=40° C.). At room temperature,it was found that the water and ethanol contact angle (θ) is 0° whereasthat of hexadecane is 90° and heptane is 70° (FIG. 4). The oilrepellency of the F-PNIPAM can be attributed to perfluoro acrylatemolecules which lowers the solid surface energy.

Referring now to FIG. 5, at an elevated temperature (T=40° C.), it wasfound that θ_(water)=90° whereas that of ethanol, hexadecane, andheptane remains relatively unchanged (θ_(ethanol)=0°,θ_(hexadecane)=80°, θ_(heptane)=50°). This indicates that the F-PNIPAMbecomes hydrophobic when the temperature is above the LCST of F-PNIPAM(which was found to be 28.7° C., as described below). Such wettabilityswitch can be attributed to the hydrophilic-hydrophobic balance in thePNIPAM network. As described above, PNIPAM is composed of bothhydrophilic (amide) and hydrophobic (isopropyl) groups. When thetemperature is below the LCST for PNIPAM, the amide group and hydrogenfrom water bond with each other. However, when the temperature is abovethe LCST, the hydrogen bond is weakened and the hydrophobic interactionbetween the isopropyl groups increases. The prevailing hydrophobicinteraction results in repelling the water when the temperature is abovethe LCST.

The wettability of F-PNIPAM copolymerized with various compositions ofF-acrylate was also studied. FIG. 6 shows a plot of the contact anglesfor various polar and non-polar liquids on F-PNIPAM surfaces withdifferent F-acrylate compositions at T=21° C. FIG. 7 is a plot of thecontact angles for various polar and non-polar liquids on F-PNIPAMsurfaces with different F-acrylate compositions at T=40° C.

Surprisingly, it was determined that the F-PNIPAM can reversibly switchits wettability to water by repeated heating-cooling cycles, whilemaintaining its oil repellency. FIG. 8 illustrates the reversiblewettability switching for water while maintaining hexadecane contactangles through repeated five cycles of heating and cooling. The F-PNIPAMis effective in the separation of polar-non-polar liquid mixtures, suchas immiscible oil-water mixture or miscible ethanol-heptane mixture dueto its selective wettability of polar liquid over non-polar liquid. Forcomparison, the wettability switch of a neat PNIPAM (that is, withoutperfluoro acrylate) surface upon temperature change was also considered.At room temperature (T=21° C.), it shows both θ_(water)=0° andθ_(hexadecane)=0°. At an elevated temperature (T=50° C.), water contactangle becomes 95° whereas θ_(hexadecane) is still 0°. This observationis similar with those reported in the previous literature.

Lower Critical Transition Temperature

The LCST of PNIPAM can be altered by co-polymerization. Copolymerizingwith hydrophilic materials typically increases the LCST of PNIPAM,whereas hydrophobic materials can result in a decrease in LCST. Variouscharacterization methods have been utilized to determine LCST ofcopolymerized PNIPAM including Differential Scanning calorimetry (DSC),cloud point, and UV-Vis spectrometry to characterize turbidity and lightscattering. Here, DSC was used to determine the LCST of the F-PNIPAM.FIG. 9 shows that the LCST of F-PNIPAM was found to be 28.7° C., whichis about 3.3° C. lower than that of a neat PNIPAM at about 32° C.). TheLCST of F-PNIPAM may be further reduced by increasing the ratio of1H,1H,2H,2H-Perfluorodecyl acrylate (F-acrylate) to PNIPAM. FIG. 9 is aDSC plot for F-PNIPAM for different F-acrylate concentrations (the topline represents neat PNIPAM, the middle line represents 5% F-acrylateF-PNIPAM, and the bottom line represents 10% F-acrylate F-PNIPAM). Theendothermic peaks, indicated by arrows in FIG. 9, show the LCST ofF-PNIPAM for each concentration of F-acrylate. It was found that theLCST decreases with increasing the concentration of F-acrylate, whichwas affirmed by the shift in the endothermic peaks to a lowertemperature. The LCST for each of the F-acrylate concentrations areshown in the table below.

F-acrylate wt. % LCST of copolymer (° C.)  0 31.8 ± 1.0  5 31.0 ± 1.0 1028.9 ± 1.0The shift in the corresponding endothermic peaks can be attributed tothe copolymerization of PNIPAM with hydrophobic F-acrylate. The lowervalue of LCST for the F-NIPAM is favorable to release and collect theabsorbed liquid (water) at a lower temperature.

Surface Energy

The Owens-Wendt method utilizes the Young's relation (Eqn. 1) and theFowke's postulation (Eqn. 2) to estimate the surface energy (γ_(SV))from the liquid contact angles.

$\begin{matrix}{{\cos \theta} = \frac{\gamma_{SV} - \gamma_{SL}}{\gamma_{LV}}} & (1) \\{\gamma_{SL} = {\gamma_{SV} + \gamma_{LV} - {2\sqrt{\gamma_{SV}^{d}\gamma_{LV}^{d}}} - {2\sqrt{\gamma_{SV}^{p}\gamma_{LV}^{p}}}}} & (2) \\{\gamma_{SV} = {\gamma_{SV}^{d} + \gamma_{SV}^{p}}} & (3) \\{\gamma_{SV}^{d} = {\gamma_{LV}\left( \frac{1 + {\cos \theta}}{2} \right)}^{2}} & (4) \\{\gamma_{SV}^{p} = {\frac{1}{\gamma_{SV}^{p}}\left\lbrack {\frac{\gamma_{LV}\left( {1 + {\cos \; \theta}} \right)}{2} - \sqrt{\gamma_{SV}^{d}\gamma_{SV}^{p}}} \right\rbrack}^{2}} & (5)\end{matrix}$

Here, γ_(SV) ^(d) is the dispersive component that accounts for thedispersive forces while γ_(SV) ^(p) is the polar component that accountsfor polar forces such as hydrogen-bond or dipole-dipole interaction. Thetable below shows the surface energy of F-NIPAM. To calculate thedispersive component (γ_(SV) ^(d)), the contact angle (OHD) and surfacetension of hexadecane (γHD=27.5 mN/m) were used in Eqn. 4. Thecalculated dispersive component (γ_(SV) ^(d)) along with the watercontact angle were used to calculate the polar component of surfaceenergy (γ_(SV) ^(p)) using Eqn. 5. Here, the dispersive and polarcomponents of water surface tension are γ_(LV) ^(d)=21.1 mN/m and γ_(LV)^(p)=51.0 mN/m, respectively. The total surface energy (γ_(SV)) ofF-NIPAM is calculated by summing up the dispersive and polar surfaceenergy components.

Wt. % of F-acrylate γ_(SV) ^(d) (mN/m) γ_(SV) ^(p) (mN/m) γ_(SV) (mN/m)0 27.5 45.2 72.7 5 8.1 5.5 13.6 10 6.9 2.7 9.5 20 6.9 3.2 10.1

Adding fluorinated materials to a surface lowers the surface energy.Therefore, the surface energy of the F-PNIPAM can be lowered byincreasing the wt. % of F-acrylate. This results in higher contactangles for contacting liquids. The hexadecane and water contact angleson F-NIPAM were used to calculate the surface energy. The surface energyof F-NIPAM decreases with increasing the F-acrylate composition, asshown in FIG. 10. At 10 wt. % F-acrylate, F-NIPAM is completely coveredwith fluorine leading to the minimum surface energy.

Absorption Tests Example 1

Absorption tests were conducted to verify that the F-PNIPAM canselectively absorb polar liquid (water) while repelling non-polar liquid(oil). First, F-PNIPAM cubes were fabricated. Subsequently, the preparedF-PNIPAM cube was completely submerged in a desired liquid bath. After 1hour, the weight of F-PNIPAM cube was measured and normalized against anas-prepared cube. FIG. 11 shows a plot of normalized uptake as afunction of hydrogel mass. Here, a normalized uptake is defined as massof absorbed liquid (water or ethanol) for 1 hour/hydrogel mass. Onlypolar liquids (water and ethanol) were absorbed by the F-PNIPAM whilenon-polar liquids (heptane) remain unchanged in their weight. Forcomparison, the absorption test was also conducted using neat PNIPAM. Itwas found that neat PNIPAM can also absorb polar liquids while repellingnon-polar liquid. Interestingly, the values of normalized uptakeincreases with increasing the total mass of cube. This is likely due tothe fact that the surface area of a cube with large mass (e.g., 200 mg)is much greater than that of a smaller cube (e.g., 40 mg), whichenhances the rapid absorption.

It was also found that the F-PNIPAM can absorb approximately 1.3 timesmore polar liquids as compared with a neat PNIPAM cube. This is becauseof a so-called ‘loosened’ NIPAM polymer network of the F-PNIPAM due tothe presence of perfluoro acrylate. Such a loosened polymer network canbe also obtained by reducing crosslinking density.

In order to verify this, multiple F-PNIPAM cubes with differentconcentration of crosslinker (MBAA) were fabricated. Here 0.5 wt %, 2.0wt %, 4.0 wt % and 6.0 wt % of MBAA were used. FIG. 12 shows a plot ofnormalized uptake as a function of MBAA concentration (wt %) in F-PNIPAMand neat PNIPAM. It clearly shows that the F-PNIPAM can absorb largeramount of water and ethanol when the crosslinker density is lower. Asimilar trend was also observed with neat PNIPAM.

Time-dependent absorption of polar liquids of the F-PNIPAM was alsoobserved. FIG. 13 shows a plot of normalized uptake as a function ofsubmerged time. For comparison, the data obtained using neat PNIPAM isalso shown. It clearly shows that F-PNIPAM quickly absorbs liquids afterit is submerged. Such a rapid absorption is also observed in neat PNIPAMtests.

It has been demonstrated that the F-PNIPAM can absorb various polarliquids such as alcohols and water whereas non-polar liquids are notabsorbed. Therefore, the F-PNIPAM may separate oil-water mixtures byselectively absorbing water. To illustrate, in a first instance, a freehexadecane and water with 50:50 vol %:vol % solution was first prepared.In the example, a total volume of the hexadecane-water mixture is about4 mL. Subsequently the F-PNIPAM is completely submerged in thehexadecane-water mixture. After approximately 80 mins, it was observedthat the F-PNIPAM can selectively absorb blue-dyed water fromhexadecane-water mixture resulting in almost complete separation asshown in FIGS. 14A, B, and C. Specifically, FIGS. 14A and 14B, showsequential photographs of the separation of free hexadecane-watermixture using a F-PNIPAM cube. The water is denser, so it falls to thebottom of the container, with the less dense hexadecane floats atop thewater. In 15B, the F-PNIPAM is swollen, having absorbed the water. In14C, the F-PNIPAM, swollen with water, is removed, leaving substantiallyonly the hexadecane.

In a second instance, a free hexadecane and water solution (50:50 vol %,total volume of about 6 mL) was prepared for separation using F-PNIPAM.The separation is illustrated in FIGS. 24A-D. In FIG. 24A, the F-PNIPAMis submerged in the 50:50 vol:vol solution. The hexadecane is less denseand therefore sits atop the water. In FIG. 24B, after 5 minutes, thewater begins to be absorbed by the F-PNIPAM. In FIG. 24C, after 15minutes, substantially all of the water is absorbed by the F-PNIPAM. InFIG. 24D, the F-PNIPAM cube is removed, leaving only the hexadecane.FIG. 25 shows a plot of separation efficiency for 50:50 vol:vol freehexadecane and water as a function of time. The separation efficiency isdefined by

${\frac{S.R.}{S.R._{eq}} \times 100},$

where S.R._(eq) indicates the equilibrium swelling ratio and S.R. is theswelling ratio at a time (t). In 15 minutes, the separation efficiencywas found to be about 99%.

To determine the separation efficiency using thermogravimetric analysis(TGA), about 16 mg of a liquid was heated from 25° C. to 105° C. at arate of 5° C./min and the temperature was held constant at 105° C. for10 minutes. FIG. 26 shows the TGA plot for remnant after free hexadecaneand water separation. The loss in weight of the remnant after theseparation was compared with the weight loss of pure hexadecane andwater to estimate the purity of the remnant. From this comparison, itwas determined that the separation efficiency for free hexadecane andwater mixture using the F-PNIPAM is about 99%.

FIG. 27 shows the time-dependent evolution of the swelling percentagewith a various submerged (contact) area of F-PNIPAM in the water phase(see inset in FIG. 27). The swelling percentage is defined as theswelling ratio (S.R.) with respect to the equilibrium swelling ratio(S.R._(eq)) i.e.

${Swelling}\mspace{14mu} \% {{= \frac{S.R.}{S.R._{eq}}}.}$

Decreasing the submerged area of F-PNIPAM results in a slower swelling.This is because water is absorbed less when the submerged area is low.Although the swelling rate is affected by the submerged area, F-PNIPAMscould effectively reach to their equilibrium swelling after 120 minutes.

Example 2

So far it has been shown that the F-PNIPAM can absorb only polar liquidswhile repel non-polar liquids. This allows for almost completeseparation of water from an immiscible oil-water mixture by selectiveabsorption. Separating immiscible liquid mixtures (oil-water) viaselective absorption using a sponge-like gel is relatively easy and hasbeen demonstrated in literature. However, the separation of miscibleliquid mixtures such as alcohol-alkane or alcohol-water throughselective absorption of one phase over another is more challenging.Unlike immiscible liquids, miscible liquids typically possess similarphysical or chemical properties.

The F-PNIPAM was also tested for its ability to separate a miscibleethanol-heptane mixture. Note that ethanol and heptane are completelymiscible. A 3 mL mixture of heptane and ethanol (2:1 vol %:vol %) wasfirst prepared, and the F-PNIPAM cube was then fully submerged for about40 min, as shown in FIG. 15A. Prior to mixing, the ethanol was dyed bluefor ease of identification. After 40 minutes, the ethanol was almostcompletely absorbed by the F-PNIPAM, as shown in FIG. 15B, leaving theclear heptane in the container. Thus, the F-PNIPAM can selectivelyabsorb ethanol from a completely miscible ethanol-heptane mixture.

Refractive index (RI) measurements were conducted to verify theseparation efficiency. The RI for a feed mixture of 1:2 vol:volethanol:heptane is 1.3761, as shown in FIG. 16. After 40 mintutes ofabsorption using the F-PNIPAM, the RI of the resulting liquid was foundto be 1.3865. This value corresponds to approximately 98 vol % ofheptane indicating that the F-PNIPAM can remove almost all ethanolthrough absorption.

Example 3

F-PNIPAM with a desired volume was prepared by molding in cubicalpolydimethylsiloxane (PDMS) mold. Briefly, the PDMS mold was prepared bymixing the main component and the curing agent in 10:1 ratio by weightfollowed by degasification in vacuum oven to remove trapped air bubbles.The mixture was then poured in a cuboidal mold of 1.2 cm×1.2 cm base andheated at 60° C. for 6 hours for cross-linking. The PDMS mold replicatedthe shape of the mold with the dimensions mentioned above. Subsequently,1 mL of the F-PNIPAM solution was poured in the PDMS mold and exposed toultraviolet light (UV-A, λ=365 nm) for 15 minutes for photocuring. Afterphotocuring, the cross-linked F-PNIPAM gel with 1.2 cm×1.2 cm×0.7 cm(about 1 cm³ volume) dimension was carefully removed from the mold.

All absorption experiments were performed using F-PNIPAM with 10 wt. %F-acrylate. The swelling ratio (S.R.) of F-PNIPAM was determined usingEqn. 6 from the weight of F-PNIPAM at time ‘t’ (W), during preparation(W_(o)) and the weight of polymer in F-PNIPAM (W_(s), equivalent toweight of dried F-PNIPAM). Similarly, the equilibrium swelling ratiowould indicate the swelling ratio of F-PNIPAM at its maximum swellingstate. The equilibrium swelling ratio (S.R._(eq)) was obtained bysubmerging F-PNIPAM in the desired solvent for seven days.

$\begin{matrix}{{S.R.} = {\frac{U}{W_{s}} = \frac{W - W_{o}}{W_{s}}}} & (6)\end{matrix}$

FIG. 17 shows the equilibrium swelling ratio of the F-PNIPAM for variouspolar and non-polar liquids. All experiments were performed at roomtemperature (T=21° C.). F-PNIPAM can absorb up to 11 times the polymerweight when submerged in polar liquids. The equilibrium S.R. for wateris around 7.42 and that for heptanol is 10.45. On the other hand, S.R.for non-polar liquids (hexadecane and heptane) are almost zero asF-PNIPAM barely absorbs them.

FIG. 18 shows the equilibrium swelling ratio of F-NIPAM for ethanol andwater as a function of cross-linker concentration. It clearly shows thatF-NIPAM absorbs less amount of liquids when the cross-linkerconcentration is increased. Absorption tests were also performed usingF-PNIPAM with various polymer mass (FIG. 19). It was found that theequilibrium swelling ratio is almost the same for F-PNIPAM withdifferent polymer mass.

Example 4

Interestingly, it was found that F-PNIPAM can absorb a larger amount ofalcohols with increasing number of hydrocarbons in alcohols, as shown inFIG. 20. For example, the swelling ratio for methanol (CH₃OH) is about6.43 while that for heptanol (C₇H₁₅OH) is 10.45. This may be explainedusing the Flory-Huggins polymer-solvent interaction parameter (χ). Theinteraction parameter χ can be related to the Hansen solubilityparameters (HSP) by Eqn. 7. The relation of χ and Hansen solubilityparameters (HSP) is useful because HSP values for common liquid orpolymers are extensively documented.

$\begin{matrix}{\chi = {\alpha {\frac{V}{RT}\left\lbrack {\left( {\delta_{D2} - \delta_{D1}} \right)^{2} + {{0.2}5\left( {\delta_{P2} - \delta_{P1}} \right)^{2}} + {{0.2}5\left( {\delta_{H2} - \delta_{H1}} \right)^{2}}} \right\rbrack}}} & (7)\end{matrix}$

If HSP values are not known, they can be estimated by using the groupcontribution method by Hoftyzer and Van Krevelen (Eqn. 8). the Hansensolubility parameters of PNIPAM were estimated as δ_(D)=19.15 √{squareroot over (MPa)}, δ_(P)=7.76 √{square root over (MPa)} and δ_(H)=7.04√{square root over (MPa)}. The Hansen solubility parameters forF-acrylate from the group contribution method were also estimated. Theestimated HSPs for F-acrylate are δ_(D)=14.87 √{square root over (MPa)},δ_(P)=2.74 √{square root over (MPa)} and δ_(H)=3.97√{square root over(MPa)}.

$\begin{matrix}{{\delta_{D} = \frac{\Sigma F_{di}}{V}},{\delta_{P} = \frac{\sqrt{\Sigma F_{pi}^{2}}}{V}},{\delta_{P} = \sqrt{\frac{\Sigma E_{hi}}{V}}}} & (8)\end{matrix}$

Using the estimated Hansen solubility parameters, the χ values of NIPAM(χ_(NIPAM)) with various alcohols can be calculated using Eqn. 7. The χvalues of F-acrylate (χ_(F-acrylate)) with various alcohols were alsocalculated. The χ values of F-PNIPAM (χ_(F-PNIPAM)) were then calculatedby considering F-acrylate a factor of 0.1 (i.e. χ_(F-PNIPAM)=0.9χ_(NIPAM)+0.1 χ_(F-acrylate)). These χ values for F-PNIPAM(χ_(F-PNIPAM)) for various alcohols are shown in FIG. 21. It is worthnoting that the correction factor α utilized in Eqn. 7 is 0.35 for allthe calculations.

Example 5

A hexadecane-in-water emulsion (30:70, vol:vol) was prepared usingsodium dodecyl sulfate (SDS) as a surfactant. SDS was dissolved in watersuch that the concentration is 10 mg/mL. Hexadecane was added to the SDSdissolved water such that the volume ratio of water and hexadecane is70:30 followed by vigorous stirring for emulsification. A cube ofF-PNIPAM (1 cm³) was submerged into 2 mL of hexadecane-in-wateremulsion, as shown in FIG. 29. After 15 minutes, almost pure hexadecanewas left indicating that almost all the water is absorbed by theF-PNIPAM cube. It was determined that the separation efficiency reachedaround 99% in 30 minutes. FIG. 30 shows the TGA plot for remnant afterseparation of hexadecane-in-water emulsion along with pure hexadecaneand water. The separation efficiency for hexadecane-in-water emulsion isabout 99%. FIG. 28 shows a plot of size distribution of hexadecanedroplets in the emulsion.

FIG. 31 shows the time-dependent separation efficiency of two differenthexadecane-in-water emulsions with different oil composition (30% and 50vol % of hexadecane). It was determined that an increase of hexadecanecomposition in the emulsions does not affect the final separationefficiency of the F-PNIPAM (about 99%). This can be attributed this tothe F-PNIPAM's resistance to oil fouling. As the F-PNIPAM isoil-repellent yet water-loving, the F-PNIPAM can effectively repel oilwhile absorbing water. This can lead to a high separation efficiencyeven for the surfactant-stabilized oil-water emulsions. It is worthnoting that separating 50:50 hexadecane-in-water emulsion is slower than30:70 hexadecane in water emulsion, which can be attributed to the factthat the contact area of the F-PNIPAM and water is lowered due to thehigh concentration of oil phases.

Example 6

A water-in-hexadecane emulsion (50:50, vol:vol) was prepared usingspan80 as a surfactant. Here, span80 was dissolved in hexadecane suchthat the concentration of span80 in hexadecane is 1 mg/mL. Water wasadded to this span80 dissolved hexadecane such that the ratio of waterto hexadecane by volume is 50:50. The mixture was then vigorouslystirred for 10 minutes to prepare the emulsion. FIG. 32 shows a plot ofsize distribution of water droplets in the emulsion.

FIG. 33 shows the separation of water-in-hexadecane emulsion (50:50,vol:vol) using the F-PNIPAM. A F-PNIPAM cube (3 cm³) is submerged inhexadecane-in-oil emulsion (6 mL). After 30 minutes, only hexadecane wasleft in the container. The separation efficiency was determined usingTGA. FIG. 34 shows the TGA plot for remnant after separation. Remnantafter separation is almost the same as that of pure hexadecane. From thecomparison, it can be determined that the separation efficiency forwater-in-hexadecane emulsion is greater than 99%. The separationefficiency was also determined by comparing the density of remnant afterseparation. The density of the remnant is 0.772 g/cm³. This isequivalent to hexadecane with less than 1 vol % water.

FIG. 35 shows a plot of time dependent separation efficiency forspan80-stabilized water-in-hexadecane (50:50 vol:vol) emulsion. Unlikethe F-PNIPAM, a neat NIPAM shows very slow separation for water-in-oilemulsions. This is because a neat NIPAM is easily fouled by oil whichmakes the water droplets difficult to be absorbed, as is describedelsewhere herein.

Example 7

The capability of the F-PNIPAM to separate miscible liquid mixtures wasalso tested. First, a miscible liquid mixture that consists of ethanol(polar) and heptane (non-polar) is separated. Heptane and ethanol aremiscible in all ranges of compositions. Here, the heptane-ethanolazeotrope (54.5 vol % heptane and 45.5 vol % ethanol) was used toeliminate the evaporation effect during separation process. FIG. 36shows the separation of the heptane-ethanol azeotrope (6 mL) using theF-PNIPAM cube (≈3 cm³). The F-PNIPAM selectively absorbs ethanol, asshown in FIG. 36B, leaving only heptane in the container (FIG. 36C).

FIG. 37 shows a plot of refractive index of heptane-ethanol mixture as afunction of vol % of heptane. The refractive index of the remnant afterseparation was 1.3864 (≈98 vol % heptane). The volume change of heptaneis negligible, while the F-PNIPAM absorbs about 2.67 mL of ethanol. Theresults indicate that the F-PNIPAM can selectively absorb ethanol fromheptane-ethanol mixture. FIG. 38 shows the heptane-ethanol azeotropeseparation efficiency as a function of submerged time.

FIG. 39 shows sequential images of a droplet of heptane-ethanolazeotrope sliding along the F-PNIPAM surface. As ethanol is selectivelyabsorbed by the F-PNIPAM, the surface of F-PNIPAM is discolored (by thecoloring of the ethanol), while the droplet becomes heptane rich.Eventually, it is possible to collect almost pure heptane after thedroplet completes sliding along the surface.

Example 8

The separation of miscible methanol (MeOH) and methyl oleate (MO) wasstudied. FIG. 40 shows the separation of the miscible MeOH-MO mixture(30:70 vol:vol) using a F-PNIPAM cube (1 cm³). Once the F-PNIPAM issubmerged in the MeOH-MO mixture (FIG. 40A), it selectively absorbsmethanol while repelling methyl oleate (FIG. 40B). After separation, theF-PNIPAM absorbs the methanol and almost pure methyl oleate is left(FIG. 40C). FIG. 41 shows a plot of separation efficiency for MeOH-MOmixture (30:70 vol:vol) as a function of submerging time. The separationefficiency reaches about 98% within 60 minutes.

Example 9

FIGS. 42A and B are plots of the volume ratio of the absorbed liquid attemperature below LCST (T=21° C.) and above LCST (T=40° C.),respectively. Here, various concentrations of ethanol-water mixtureincluding 10:90, 50:50 and 96.5:3.5 (ethanol:water (vol:vol)). In thegraphs, the bar on the left represents ethanol, while the bar on theright represents water. Of note, the composition 96.5:3.5 (ethanol:water(vol:vol)) is the azeotrope of ethanol-water mixture. When thetemperature is below LCST, the volume ratio of ethanol in the absorbedliquid is significantly higher than the initial ethanol composition inthe feed mixture. For example, the volume fraction of ethanol in theabsorbed liquid is 99.59 vol % although the initial ethanol compositionis 50 vol %. This is attributed to to χ_(EtOH)<χ_(water) (χ_(EtOH)=0.43and χ_(water)=0.45). Because χ_(EtOH)<χ_(water), F-PNIPAM absorbs highervolume of ethanol from the ethanol-water mixture. Moreover, when thetemperature is above LCST, F-PNIPAM absorbed only ethanol whilerepelling water. The table below shows the volumes of ethanol and waterabsorbed by the F-PNIPAM below and above LCST for various ethanol:waterratios. Negative numbers indicate release of the Oven liquid.

Ethanol:water (vol:vol) 10:90 50:50 96.5:3.5 (Azeotrope) Below LCSTWater absorbed (μL) 41 3 −360 (T = 21° C.) Ethanol absorbed (μL) 29 7301,920 Above LCST Water absorbed (μL) −194 −183 −330 (T = 40° C.) Ethanolabsorbed (μL) 29 785 1,840

Example 10

The F-PNIPAM was also used to separate a polar-polar liquid mixtureconsisting of dimethylformamide (DMF) and water. The χ value of theF-PNIPAM with DMF was determined to be 0.22. Since χ_(DMF)<χ_(water),(χ_(water)=0.45), it is expected that the F-PNIPAM can absorb a largeramount of DMF than water. FIGS. 43A and 43B are plots of the volumeratio of the absorbed liquid at a temperature below LCST (T=21° C.) andabove LCST (T=40° C.), respectively. The table below shows the volume ofDMF and water absorbed by F-NIPAM below and above LCST.

DMF:water (vol:vol) 10:90 50:50 90:10 Below LCST Water absorbed (μL) 67035 −500 (T = 21° C.) DMF absorbed (μL) 90 608 1,663 Above LCST Waterabsorbed (μL) −100 −209 −673 (T = 40° C.) DMF absorbed (μL) 91 605 1,979

Absorption Kinetics

To study the kinetics of absorption, the F-PNIPAM was submerged in thedesired liquid and the weight change was recorded at intervals. FIG. 22shows a plot of the swilling ratio of the F-PNIPAM in water as afunction of submerging time. The absorption behavior can be explainedwith the first order kinetics. Here, the S.R. at a given time (t) isgiven by S.R.=S.R._(eq) (1−e^(−k,t)) where S.R._(eq) is the equilibriumswelling ratio and k_(s) is the swelling rate constant. The rateconstant for water is found to be 0.0023 sec⁻¹. FIG. 23 shows the plotsof S.R. values for various alcohols as a function of submerging time.The k_(s) are found as 0.0014 sec⁻¹, 0.0015 sec⁻¹, 0.0025 sec⁻¹, 0.0040sec⁻¹, 0.0040 sec⁻¹, and 0.0040 sec⁻¹ for methanol, ethanol, butanol,pentanol, hexanol and heptanol, respectively.

Recovery Experiments Example 11

It was previously described that the F-PNIPAM can selectively absorbpolar liquid (water) from a non-polar liquid (hexadecane, oil).Releasing the absorbed water from F-PNIPAM is critical to recover thewater and to recycle the F-PNIPAM for further separation operation.

One of the simplest methods to release water from hydrogel is heattreatment. PNIPAM releases water at a temperature above its LCST,discussed further below. Although heat treatment is effective to releaseand recover water, it is highly energy-intensive. Mechanical compressionhas been utilized to squeeze water from hydrogel. However, this methodcan also be limited by damaging or sometimes disintegrating hydrogel.These challenges associated to the current thermal and mechanicaltreatments may be overcome by utilizing thermodynamic approaches. Twodifferent techniques were tested to release water from F-PNIPAM. One isso-called co-nonsolvency and the other is osmosis.

Co-nonsolvency refers to a finding that hydrogel can release containingliquid when it is submerged in a solution of two or more liquids. Forexample, PNIPAM releases water when it is submerged in an ethanol-watermixture having a certain composition. A solution of ethanol and water(1:1) was used to test releasing water of the F-PNIPAM. To test therelease of water of the F-PNIPAM, it was introduced to a water bath for1 hour. The F-PNIPAM swelled and absorbed about 1,473 mg. Subsequently,the F-PNIPAM was transferred into a 15 mL of ethanol-water (1:1)solution bath. After 1 hour, the F-PNIPAM had released 1,234 mg ofwater. This value corresponds to about 83.8% of that water which wasabsorbed. FIG. 44 shows photographs of the F-PNIPAM after absorbingwater and releasing 91% of water after being submerged in ethanol-water(1:1) solution.

In another embodiment, osmosis (or osmotic pressure)-driven methods wereused for releasing water from hydrogel. Here, a sodium chloride aqueoussolution (NaCl, 300 mg/mL) was used to release water from F-PNIPAM. TheF-PNIPAM containing 1,200 mg of water is submerged in NaCl solution for1 hour. 870 mg of water was released, which corresponds to about 71.8%of that which was absorbed.

Example 12

A combined approach—e.g., submerging the F-PNIPAM in a 1:1 ethanol:watersolution containing NaCl—may result in an enhanced release. Therefore atest was conducted using 1:1 ethanol:water solution containing 30 mg/mLof NaCl. Surprisingly, it was observed that the F-PNIPAM lost about93.3% of water in 1 hour, which indicates that almost all water isreleased. The table below summarizes the experimental data representingthe releasing of the absorbed water from F-PNIPAM using differentsolutions.

A. Absorbed water B. Released water C. Released water % recoveryRecovery solution (mg) after 30 min (mg) after 1 hour (mg) (=C/A × 100)NaCl (aq) 1,200 861 870 71.8% 1:1 Ethanol:Water 1,473 1,087 1,234 83.8%1:1 Ethanol:Water 1,469 1,216 1,371 93.3% with NaClFor comparison, the same recovery tests of a neat PNIPAM were alsoconducted. The table below summarizes the results.

A. Absorbed water B. Released water C. Released water % recoveryRecovery solution (mg) after 30 min (mg) after 1 hour (mg) (=C/A × 100)NaCl (aq) 1,059 519 515 48.6% 1:1 Ethanol:Water 1,430 1,019 1,126 78.7%1:1 Ethanol:Water 1,139 996 1,029 90.3% with NaCl

As discussed in greater detail above, the F-PNIPAM can also selectivelyabsorb ethanol from a completely miscible mixture with a non-polarliquid (heptane). The same solutions discussed above were used torecover ethanol from the F-PNIPAM. The table below summarizes theresults.

A. Absorbed B. Released ethanol C. Released ethanol % recovery Recoverysolution ethanol (mg) after 30 min (mg) after 1 hour (mg) (=C/A × 100)NaCl (aq) 1,580 285 305 19.3% 1:1 Ethanol:Water 1,440 207 157 10.9% 1:1Ethanol:Water 1,460 252 329 22.5% with NaClFor comparison, the same recovery tests of a neat PNIPAM were alsoconducted, and the table below summarizes the results.

A. Absorbed B. Released ethanol C. Released ethanol % recovery Recoverysolution ethanol (mg) after 30 min (mg) after 1 hour (mg) (=C/A × 100)NaCl (aq) 1,010 195 217 21.5% 1:1 Ethanol:Water 1,332 187 270 20.3% 1:1Ethanol:Water 1,171 219 201 17.2% with NaCl

The recyclability of the F-PNIPAM was observed after releasing water bysubmerging it in a water bath for 1 hour and comparing the amount ofabsorbed water with that obtained using an as-prepared F-PNIPAM, asdescribed above. FIG. 45 shows that the F-PNIPAM after releasing watercan absorb almost the same amount of water, which indicates that theF-PNIPAM can be repeatedly used in the separation of liquid mixtures.FIG. 46 shows that the F-PNIPAM after releasing ethanol can also absorbalmost same amount of ethanol.

Example 13

Due to the thermo-responsive behavior of NIPAM, the F-PNIPAM can releaseabsorbed liquid (water) at a temperature above the LCST. It should beemphasized that the LCST for the F-PNIPAM was found to be about 28° C.This allows for water release at a mild heat treatment. FIG. 47 showsthe recovery of water at T=33° C. It was determined that F-PNIPAM canrelease 1,150 mg of water. This is equivalent to the recovery of about82% of the absorbed water. It is worth noting that the LCST of NIPAM canbe controlled by copolymerizing with other polymers. For example, addinga low surface energy material such as F-acrylate considered in thisstudy can lower the LCST.

Example 14

Salt ions may induce deswelling of NIPAM and consequently, the releaseof absorbed liquid. Anions contribute to the deswelling process. First,anions can polarize the water molecules that hydrogen bond to the amidegroups. This results in the weakened hydrogen bond. In addition, anionsdisrupt the hydrophobic hydration of water molecules to the isopropylgroups.

Sodium chloride (NaCl) was found to effectively induce deswelling of theF-PNIPAM and consequently releasing the absorbed liquid. FIG. 48 shows aplot of % water recovery of the F-PNIPAM submerged in aqueous NaClsolution. Here two different concentrations (50 mg/mL and 100 mg/mL) ofNaCl were used. About 98% of the absorbed water can be recovered bysubmerging the F-PNIPAM in NaCl solution for 120 minutes. The recoveryof water using aqueous salt solution follows the first order kineticsgiven as Recovery %=(1−e^(−k,t))×100. Here, k_(r) is the recovery rateat a given time (t). The first order kinetic model matches well with theexperimental data with k_(r)=0.0048 and k_(r)=0.0011 for 50 mg/mL and100 mg/mL, respectively.

It was also demonstrated that ethanol can be released using an aqueousNaCl solution. FIG. 49 shows a plot of the percentage of ethanolrecovery for the F-PNIPAM submerged in aqueous NaCl solutions with twodifferent concentrations (50 mg/mL and 100 mg/mL). Again, about 98% ofethanol can be recovered. The first order kinetics model describes wellthe experimental data with k_(r)=0.0005 for 50 mg/mL and k_(r)=0.0007for 100 mg/mL.

Fouling of Neat NIPAM

Hydrophilic yet oleophobic materials have been used in separation ofliquid mixtures consisting of polar (such as water) and non-polar (suchas oil) phases. For example, hydrophilic/oleophobic membranes canselectively allow water to wet the surface and permeate through whilerepelling oil. Similarly, it has been demonstrated that the F-PNIPAM asdescribed herein can be preferentially wet by water while repelling oilat temperatures below the LCST. Specifically, it was demonstrated that awater droplet can undercut the oil and consequently wet the surface.Such self-cleaning ability is critical to mitigate surface fouling.

By contrast, FIG. 50 shows a neat NIPAM (without F-acrylate) that isfouled by oil. Here, an oil droplet (hexadecane, dyed red) withsurfactant (span80, 5 mg/mL) can easily spread on the surface. When awater droplet (dyed blue) comes in contact, oil hinders the water fromwetting the surface and being absorbed. Therefore, it is difficult toclean the oil fouled NIPAM surface by applying water.

Apparatus for Separation and In-Situ Recovery of Absorbed Liquid UsingF-PNIPAM

According to another embodiment of the invention, an apparatus wasdeveloped for separating and recovering absorbed liquid from theF-PNIPAM. In order to achieve the continuous separation and simultaneousrelease of the absorbed liquid, the F-PNIPAM needs to contact a liquidmixture to selectively absorb one phase over the other while contactingthe salt aqueous solution to release the absorbed liquid.

Referring now to FIG. 51, the apparatus for continuous separation ofoil-water mixtures and the simultaneous recovery of the absorbed waterincludes a first container for holding a salt solution (e.g., a solutionof NaCl), and a second container for holding the F-PNIPAM and themixture to be separated. The F-PNIPAM is situated in the secondcontainer such that it is in continuous contact with the NaCl solution,as shown in FIG. 51A. Upon application of the oil-water mixture on topof the F-PNIPAM, water from the mixture is continuously absorbed by theF-PNIPAM, and at the same time, the absorbed water is released to theNaCl aqueous solution bath, while the oil is repelled. Utilizing thisapparatus, the continuous oil-water separation is illustrated in FIGS.51B and 51C. Here, the water was dyed blue, and the oil was dyed red.The release of the absorbed water can be verified by the color change inthe NaCl solution bath from clear to blue, while the red oil remains inthe second container atop the F-PNIPAM.

FIG. 52 illustrates another embodiment of an apparatus for separating aflowing oil-water mixture. Here, as shown in FIG. 52A, the F-PNIPAM iscrosslinked as a cylindrical hollowed tube such that it can contact thesalt solution (here, an aqueous NaCl bath). The oil-water mixture flowsthrough the F-PNIPAM tube, and only water is absorbed by the F-PNIPAMwhile the oil slides off the surface and is collected at the end of thetube. At the same time, the absorbed water will be released into theNaCl bath underneath the F-PNIPAM. FIGS. 52B and 52C show the continuousseparation of oil-water mixture. The F-PNIPAM tube may be inclinedslightly to allow the introduced oil-water mixture to slide through thehollow F-PNIPAM. Here water was dyed blue whereas oil was dyed red. Asthe NaCl bath was initially colorless, the release of the absorbed water(dyed blue) can be clearly observed by the change of color of the NaClbath.

Thus has been described a new hydrogel, F-PNIPAM, and methods of makingsame, and apparatus for separating absorbed liquids from the hydrogel,which have superior qualities when compared to prior art hydrogels.While specific examples are provided herein in describing the invention,it shall be understood that the example are only for the purpose ofdescribing the invention, and are not intended to be limiting. Manydifferent arrangements of the various components depicted, as well ascomponents not shown, are possible without departing from the spirit andscope of the present invention. Embodiments of the present inventionhave been described with the intent to be illustrative rather thanrestrictive. Alternative embodiments will become apparent to thoseskilled in the art that do not depart from its scope. A skilled artisanmay develop alternative means of implementing the aforementionedimprovements without departing from the scope of the present invention.It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations.

1. A hydrogel, comprising effective amounts of: N-ispropylacrylamide (NIPAM); N,N′ Methylenebisacrylamide (MBAA); 2-hydroxy-2-methylpropiophenone; and 1H,1H,2H,2H-perfluorodecyl acrylate; wherein the hydrogel has the following chemical structure:


2. The hydrogel of claim 1, wherein the weight ratio of the N-isopropylacrylamide, N,N′ Methylenebisacrylamide, and 2-hydroxy-2-methylpropiophenone is 97:1:2
 3. The hydrogel of claim 1, wherein the hydrogel is selectively wetted by polar liquids.
 4. The hydrogel of claim 1, wherein the wettability to polar liquids of the hydrogels is reversible based on a temperature of the hydrogel.
 5. The hydrogel of claim 4, wherein the wettability to nonpolar liquids of the hydrogels is unchanged based on the temperature of the hydrogel.
 6. The hydrogel of claim 1, wherein the hydrogel is recyclable by submerging the hydrogel containing a polar liquid into a solution containing equal amounts of ethanol and water and an effective amount of sodium chloride, wherein after about one hour in the solution, the hydrogel is almost completely devoid of the polar liquid.
 7. The hydrogel of claim 1, wherein the hydrogel is effective to selectively absorb a polar liquid from a miscible solution comprising the polar liquid and a nonpolar liquid
 8. A method for synthesizing a hydrogel, comprising the steps of: (a) dissolving N-isopropylacrylamide, N,N′ Methylenebisacrylamide (MBAA) and 2-hydroxy-2-methylpropiophenone in deionized water to form a NIPAM solution; (b) preparing a solution of 1H,1H,2H,2H-Perfluorodecyl acrylate in ethanol; (c) separately stirring the solutions prepared in steps (a) and (b) for approximately three hours; (c) gradually introducing the solution from step (b) into the solution from step (a); (d) stirring the resulting solution from step (c) for about two hours; (e) pouring the resulting solution from step (d) into a mold and irradiating with UV-A (λ=365 nm) for about 15 minutes.
 9. The method of claim 8, wherein the weight ratio of the N-isopropylacrylamide, N,N′ Methylenebisacrylamide, and 2-hydroxy-2-methylpropiophenone is 97:1:2.
 10. The method of claim 9, wherein the overall concentration of the solution in step (a) is 200 mg/ml.
 11. The method of claim 10, wherein the overall concentration of the solution in step (b) is 200 mg/ml.
 12. The method of claim 11, wherein the weight ratio of 1H,1H,2H,2H-Perfluorodecyl acrylate to the NIPAM solution in step (c) is 9:1.
 13. The method of claim 8, wherein the weight percent of MBAA is between about 0.1 and 10%.
 14. The method of claim 13, wherein the weight percent of MBAA is between about 0.4 and 0.6%.
 15. A method for synthesizing a hydrogel, comprising the steps of: (a) dissolving effective amounts of a monomer, a crosslinker, and a photoinitiator in deionized water, wherein the overall concentration is about 200 mg/ml; (b) preparing a solution of a fluorinated acrylate or diacrylate in ethanol wherein the overall concentration is about 200 mg/ml; (c) separately stirring the solutions prepared in steps (a) and (b) for approximately three hours; (c) gradually introducing the solution from step (b) into the solution from step (a); (d) stirring the resulting solution from step (c) for about two hours; and (e) exposing the solution from step (d) to UV-A irradiation for about 15 minutes.
 16. The method of claim 15, wherein the monomer is N-isopropylacrylamide, the cross-linker is N,N′ Methylenebisacrylamide, and the photoinitiator is 2-hydroxy-2-methylpropiophenone
 17. The method of claim 16, wherein the fluorinated acrylate or diacrylate is selected from the list consisting of: 1H,1H,2H,2H-Perfluoroecyl acrylate; 1H,1H,6H,6H-Perfluoro-1,6-hexandiol diacrylate; 1H,1H,-perfluoro-n-octyl methancrylate; and 1H,1H-perfluoro-n-octyl acrylate.
 18. The method of claim 15, wherein step (c) occurs in the dark.
 19. The method of claim 15, wherein the solution from step (d) is formed into a mold.
 20. The method of claim 15, wherein the hydrogel is effective for separating miscible and immiscible solutions. 